CA1182811A

CA1182811A - Thiolated polypeptide compound derived from a tetanus toxin fragment, the process for its obtention and its applications

Info

Publication number: CA1182811A
Application number: CA000394587A
Authority: CA
Inventors: Bernard Bizzini
Original assignee: Institut Pasteur de Lille
Current assignee: Institut Pasteur de Lille
Priority date: 1981-01-22
Filing date: 1982-01-21
Publication date: 1985-02-19
Also published as: ES8400736A1; ES508935A0; EP0057140A2; US4479940A; ES519456A0; ES8306714A1; DE3264703D1; EP0057140B1; JPS57176916A; FR2498192A2; EP0057140A3; FR2498192B2

Abstract

ABSTRACT OF DISCLOSURE
A new thiolated polypeptide compound derived from a fragment of tetanus toxin, the process for its obtention and its applications.
This compound consists of the IIc fragment of tetanus toxin,having at least one. -SH group either directly or Indirectly bound thereto. It is usable as a specific neuropharmacological transport agent for transporting a medicine to the central nervous system,as a specific labeller for neuronal cells or or diagnosis purposes. It can be coupled with a medicine or a labeller.

Description

The pres~nt invention relates to a new thiolated polypeptide compo~d derived from a tetanus toxin fragment, to the process for its obtention and its applications.
With regard to the tetanus toxin i-tself or its anatoxin, it was already proposed to use them notably for obtaining vaccines or assay reagents. In order to illustrate this prior art following re~erences will be cited:
-FR patent for addition n 74,16,936 published under n 2,270,891, which relates to a process for obtaining vaccines by txeatment of a toxic product with glutaraldehyde. This process consists in treating a ~oxic product w.ith glutaraldehyde in effecting the polymerisation of a llmited number of molecules of said product and the detoxification of said product. In this process ~etanus toxin may be used as toxic product.
-FR patent appl iGat ion n 77,29,186, published under n 2~366,569, relates to an immunochemical process for assaying haptens, wherein are used a particle sPnsitized by an antibody, prepared by sensitizing fine particles by an antibody of the hapten to be assayed and a hap*en-carrier conjugate. The carrier of this conjugate may be notably te~tanus toxoid.
This hapten-carrier conjugate is used as reagent in an immunochemical process and also for immuniza-tion of an animal in order to obtain corresponding antibodies Csee on page 4, lines 20 to 32J .

,~

The tetanus toxoid is therefore used as a carrier o~ the hapten in the body of the animal for obtaining antibodles. However,there exists no teaching in this FR patent about a particular -Fragment o~ tetanus toxin and its possible use as axonal transport agent ~or drugs.
-GB patent 2,013,690 relates to an antigen for early pregnancy test and contrareptlve vaccine This antigen is obtained from the ~-subunit of human chorionic gonadotrophin by reducing and cleaving of three, four, five or six o~ intrachain disulphide bounds of said ~ -subunit,alkylating of the thus reducing intrachain disulphide groups and isolating of the produced antigen. This antigen may be coupled with a protein or a hapten to enhance its immunological specificity. Tetanus toxoid is cited as sultable protein. --GB patent 1,492,445 relates to a composition comprising a conJugate of a subJect-compatible immunogenic oarrier and an immunochemically purified hormone derivative. The tetanus toxoid is used as carrier in this composltion.
-DE-OS patent application 1,617,880 relates to a process for obtaining bioactive organotrope ~-- ~=~~ --substances,particularly drugs. This process consists in making a oon~ugate ofa biologically active substance with organotrope receptive substances obtained ~rom cellular membranes or antibodies. The toxins may be used as organotrope substances.

Furthermore,it was proposed to use thiolated proteins as drug carriers. Reference may be made to US
patent 3,171,831 which relates to thiolation of proteins by reaction with homocysteine thiolactone in the presence S of ~ tertlary amine. The ~hiolated pro-teins thus obtained, for example gelatin,may be used as carriers f'or drug~.
According to example 18 of this US patent 3,171,~31 the gelatin thus treated is used ~or encapsulating a pharmaceutical product which is sensitive to the acid environment o~ the stomach. The pharmaceutical product is therefore ~n this case not coupled with the thiolated protein but coated with it.
On the other hand,it was disclosed in FR
patent appllcation 76~37J367 published under n2,33~,95~ a reagent for immunoenzymatic determin~tion.
~his reagent is composed by an antigen an~d an enzyme coupled by means o~ an ester of maleimidobenæoic acid and n-hydroxy-succinimide.
It iS known that tetanus toxin is retrogradely $ransported to the central nervous system and the periph~ral nervous ~ystem. In this respect,reference may be made to the article of BIZZINI et al entitled:

. ..~
:'An antlgenic polypeptide ~ragment isolated fr-om tetanus ~
toxin chemical characterization,binding to ganqliosides and retrograde axonal transport in various neuron systems",which appeared in the "~ournal of Neurochemistry", 1977,vol.28,pp 529~542,and to all the bibliographic references cited in this article.

Various studies have shown tha-t tetanus -toxin may be degraded or cleaYed into several Fractions or subunits, For example, COHEN ~t al ! The ~ournal of Immunology vol.104,n6 ~une 1970 7have shown that the freezing- thawing of the cr-lde filtrate of Clo tridium tetani culture results in a degradation of ~.
the molecule of tetanus toxin;the resul-ting degraded tetanus toxin is practically devoid of toxicity and has a flocculating power lower than the one of tetanus toxin.

BIZZINI and RAYNAUD have also stud~ed the subunits A-I, A-II,A-III and 8-I, B-II and B-III of tetanus toxin. ! C.R. Acad, Sc. Paris, Te27~1974 series D,pp.1809-1811 and Annales of Pasteur Institute Paris 126 , 159-176 ~1975) 7. French patent 74,36,622 (publication 2,249,679) discloses an immunogenic atoxic produot obtained from tetanus toxin.Thls atoxic product is obtained by the treatment of tetanus toxin with a proteinase.
MORRI5 et al.in The 30urnal of Biological Chemistry volO255 n13 ~uly 10,1980 ppl6071-6076 disclose the iLnteraction of frayments B and C of Tetanus toxin with neural and thyro;d membranes and with gangliosides. It i5 notably noted i--n this article that fragment C ,obtained by papain digestion at 55~C,undergoes ~5 retrograde transport in a manner slmilar to that of intact tetanus toxin3whereas fragment B does-not similarly accumu-late in the first distal seg~ent to,th~e, ligation,ibut does appear to be taken up or internalized in a diffuse pattern~'(se~e on-page 6075,left-column~.

The principal physico~chemical proper-ties o~
the dif~erent subproducts of tetanus -toxin are given on page 230 of ar-ticle entitled "Tetanus toxin"

of ~IZZINI in Microbiological Reviews , ~une 1979,p~2?4-

2~0 and notably those of fragment C and fragment II which is obtained ,by papain digestion a-t a temperature a~ which no formation of fragment C occurs.
BIZZINI et al have also isolated from frozen crude toxin a polypeptide fragment of the toxin whlch is identical,from ~he immunologioal point of Yiew, to the above mentloned frag~ents A_II and B-II,but differs there~rom by its size and toxicity L see in this respect 30urna~ of Neuroohemistry,1977 3 vol ~ 28,pp.529-542 7.
This fragmentynamed ~-IIb ,is capable of binding to the gangliosides and to the synaptic membranes with an affinity which is even greater than that of tetanus toxin.
French patent applicatlon n79~29r289,discloses a ne~ thiolated peptide compound derived from tetanus toxin,the process for lts obtention,and its applications.
This ~hiolated polypeptide compound consists of the 8-IIb fragment of tetanu toxin having bound thereto at least one ~SH group;this compound substantlally exhibits the same properties of axonal retrograde transport and of binding to the tetanus ~oxin receptors as the B-IIb fragment itself. This thiolated polypeptide compound is suitable as a neuroPharmacologic3l transport agent for conducting to the central ner~ous system pharmacological or chemotherapeutic agents. This polypeptide agen~ is also agent suitable as a label~ng /. neuronal cells. The properties of thi thiolated polypeptide compound are also described in Brain Research 193(1980)221-227.

There has be~n now ~ound a new ~hiolated polypep~ide compound derived ~rom ano~her tetanus toxin fragmentgwh.ich may be al~o u~ed as a neuropharmaoological transport agent to transport pharmacological or 5 chemotherapeutic agen~s to th~ gentral nervou system.
In one aspect of the invention there is provided a process for thiolating IIC fragment of tetanus toxin comprising reacting the fragment with a thiolating agent.
The thiolating agent and reaction conditions are selected such that the thiolated fragment includes one or more Z-SH groups, preferably between 1 and 5 Z-SH groups lZ being the residue of the thiolating agent), has retrograde axonal transport properties, and is capable of binding to the specific receptors of the tetanus toxin in the central nervous system.
In a further aspect of the invention, there is provided a process for producing a conjugate of thiolated IIC fragment of tetanus toxin and a medicine having NH2 groups comprising reacting the medicine with a compound capable of introducing into the medicine dithiopyridyl groups or with a compound capable of introducing into the medicine maleimido groups, and reacting the resulting product with the thiolated fragment. Preferably, the resulting conjugate comprises between 1 and 5 disulphide bonds.

The pre~n~ lnvention therefore relate~ to a new thiolated polypeptide compound,its ~btention and its applications The thiolated polypeptide compound acoording to the invention is the IIC fragment of tetanus toxin having bound thereto at least one -SH group and substantially exhibiting the same properties of axonal retrograde transport and of binding to ~he tetanus ~oxin receptnrs as the IIC fragm~nt it~elf.
The B-IIb ~ragment of tetanus toxin is derived from a ~rozen filtrate of a culture of (Harvard strain n6037 of Collection NationalQ de Cultures de Microorganismes de l'In-titut Pasteur in PARIS7FRANCE)by ultrafiltration for re~oving the substances w.Lth a molecular weight lower than lO,000, then by fraotionating by means of ammonium sulfate and gel ~iltration.
A number of fragment~ slmllar to the B-IIb fragment may be obtained by papain diges~ion of the whole tetanus toxin or nf the heavy chain thereof ~beta fragment).
In this connection,reference may be made to the following articles:

-7a-L~

-BIZZINI, B. and RAYNAUD,M. entitled "Immunological and biologioal study of subunits of tetanus toxin"
,27~,series D (1974)1809-1812.
~BIZZINI, B., International Symposium on Venoms and Toxinq, March 4-611977,~ombay (India);abstract published in Toxicon (1978),volume 15,p.141~
-BIZZINI B., Tetanus toxin ~1979 )224~2~0.
The IIC fragment is obtained by digestion of the purified tetanus toxin by means of papain insolublli2ed on the gel, "Sepharose 4B" ,activated with cyanogen bromide accordlng to the procedure disolosed in the pre~iously mentioned Compte Rendu de l'Académie des Sciences / T.279 December 1974; series D-1809 7,except that digestion is effected only for 4 hours.
S.t.atement will be given hereinafter of the prooedure to obtain the IIo fragment used as a starting oompound for providing the polypeptide compound according to the inventionO There was used the puri~ied tetanus toxin derived from a filtrate of a 5-day oulture of Clostridium tet obtained by the process disclosed by:
-BIZZINI, B TURPIN A., and RAYNAUD,M. in the article entitled~-Production and purification of the tetanus toxin ~ 116(19~9)~8~~712.
.
The purlfied ~etanus toxln thus obtained assayed 315~ Lf (L1mes floculationis or Floculatlon unit) per mg of N and 1,2.10~ MLD (Minimum lethal dose) per mg of N.

120 mg of the purlfied tetanus toxln were treated for 4 hours at 37C in a water bath,with 200 units of insolublized papain,under occasional atirring.
At the end of the digestion period,the papain was immediately removed by centrlPugatlon for 5 minutes at 10000 xg in a cen~rifugal machine refrigerated to 4C.
The cupernatent was filtrated on a column of "Sephadex G-100"(2.5 x 80 cm) buffered wikh 0.1 M Tris9 HCl buffer, pH ~9eontaining 1 M of NaCl. The effluent was oontinuously controlled at 280 nm and collected in 2 ml-fractions. The fractions corresponding to the peak, having a molecular weight close to 4~000,were combined together. The volume of the oombined fractions was reduced to 2 ml before filtration on a oolumn (2.5 x 80 cm) of polyacrylamide-agarose gel known under the trade name "Ultrogel AoA 54",using the same buffer. The fractions representing the maln peak were combined together and the re~ulting volume was reduoed to 3 ml by dialysis against a solution oontaining 50% of polyethylene glycol 20~000 (sigma ).
The undigested toxin or the IbC fragment liable to contaminate the II~ fragmen-t were adsorbed on an -- affinity column of "Sepharose" gel acti~ated with cyanogen bromide which previously had coupled thereto,covalently,the IgG ~raction of an anti-Ibc serum. The unadsorbed ~raetion present in the filtrate constitutes the IIo fragment~Thic fragment is not toxic to mice at a dose o~ 1 mg at one time.

:~ d.~ 9~
The afPlnity column wa~ prepared aocording to the following procedure. The IgC fraction was isola~ed by precipitating the anti-Ib~ fragment antiserum. The anti-fragment I9G fraction assayed ~S00 Lf per ml. The IgG fraction was coupled to "Sepharose 4B" activated with cyanogen bromid~ according ko the procedure disclosed by Wilchek et al. "A general method for the specific lsolation of peptides containing modified residues,using insoluble antibody columns. ~ "
The IIC fragment has the following physico~
ohemical properties:
-moleoular weight of about 46,000 -~interacts with gangliosides -it is free of toxicity -it interacts with synap~iC membranes isolated from rat spinal cord~
The immunological properties of the IIC fragmen$
are as follows:
the IIc fragment give rise to a cross reaction with the tetanus toxin and the heavy chain ( ~ fragment) thereof. The IIC fragment gives rise to an identity reaction with the B-IIb fragment;the IIC fragment ~~~~ ~~ ~~-gives rises to a non-identity reaction with the I~c fragment and the light ohain (~ fragment).
The main characterist~ os of the IIC fragment are summarized in the artioles by B. BIZZINI(Microbiological Reviews, ~une 1979,pp.224_240 and Toxic~ ~1978),Vol.15, p.l41)wherein it is stated that the fragments similar to the B-IIb ~ragment may be obtained by dlgestion by mean~ of papain and that tho e fragmentc probably contaln the portion of the heavy chain which is Involved in the binding o~ the toxin molecule to lts receptors in the nervous oell. On the other hand9it is stated that the tetanus toxin fragment~,which tlll have the ability to bind wi~h gangliosides should undergo axanal retrograde transport as the whole toxin does.
It was now found that the II~ fragment may be bound to synaptic membranes and could undergo axonal retrograde transport to the central nervous system. The thiolated IIo ~ragment,which i-~ the obJect of the present invention,will also undergo axonal retrograde transport to the central nervous system and will bind to the synaptic membranes. Since in this field any chemical modifioatlon might inhibit the pharmaoological properties o~ a fragment,it wa~ not at all obvious9even in view of the teachings relating to the B-IIb fragment,that the thiolated polypeptide compound according to the inven-tion could also be used as an axonal retrograde carrier for pharmacological or chemotherapeutic agents or as a diagnosis reagent,either alone or as associated with another substance adapted to evidence,for example,a specific antigen in the central nervous system.
The present invention therefore relates to a new thiolated polypeptide compound consisting of the IIC fragment bearing at least one -SH group,suitable especially as a neuropharmacological transport agent and as a specific labelling agent for neuronal cells.

Similarly to the proc~ s for obtaining the thiolated polypeptlde compound according to FR patent 79~ 29 /289,the -SH group or groups iQ or are direotly or indirectly bound to the II~ fragment In general,taking into account the process for it.~ obtentlon3which involYes a thiolation,the binding of the -SH g~oup~ will occur through ~he residue of the thiolation agent. Be~ides,the latter i~ bound to the IXc fragment through the -NH2 groups oarried thereby.
The thiola~ed polypeptide compound accordlng to the present invention is produced by thiolation of the IIC
fragment obtained by the above described process, The thiolation of the IIC ~ragment can be oarried out by conventional means permitting ~he introduction of -SH groups on a moleoule ¢omprising amino groups,but for the purpo~es of the invention,the means in yuestion should not denature the properties of axonal transport and of bind~ng to the specific receptor3 of the tetanus toxin in the central nervous system,of the II~ ~ragment.
~y way of example,~t will be mentioned that the thiolation of the IIC fragment oan be achieved with ~he ~ollowing thiolation ayent~.
4-methyl-mercaptobutyrimldate:HS-~ CH2)3-C8-CH3 NH +Cl-(Biochemistry vol.l7 N8,1978) 2-iminothlolane (Sohramm Ho~ and Dolffer T.(1977 Z. Physiol. Chem.358 r 137-139 ) ~

N-acetylhomocysteine thiolaotone (AHT)(see ~.Am.

Chem. Soc.1960,~2 , 565-571~

~\
5H 2 C C--~13 C CH

S acetyl-meroaptosuooinic anhydride (AMS)(~,Am.Chem.
10~oc~1959,~1 , 3802~3803) CH3-CO-S-CH - C ~
>

O

On the other hand,it will be mentioned that the known processes of thiolation consisting of a dithiopyridylation step and a reduotion step are unsuitable for the purposes of the invention. Indeed,the properties of axonal transport and o~ binding of the thus thiolated IIC
fragment are modified in the csurse of the reduction step.
For example,the thiolatlon eFfected by reaction with the N-~uccinimidyl-3-(2-pyridyl-dithio)-propionate and by reduotion of the dithiopyridylated compound ~o obtained 9 ~or example according to the procedure described by CARLSSON et al ! Bioch. ~. (1978)173 ~23-724 7is unsuitable ~or the purposes of the invention.

In order to be more preci~e,it will be stated that the thiolated polypeptide csmpound according to the invention aomprises one or more Z-SH groups,~n which Z
is the residue of the thiolat~on agent.
Thus,if one of the thiolation agents mentioned above i~ employed 9 Z-SH then represents:
NH ~ Cl~
Il -C- (CH23 3 -SH; -CO - CH - CH2 - CH2-SH, HS-CH-CO-;
NH - CO - C~13 C~12-COOH
-C -(CH2)3 -SH
NH
The thiolation o~ the IIo fragmen~ is achieved on the NH2 groups thereof.
It has been Found that the thiolated polypeptide 15 compound according to the invent~on is sultable as a neuropharmacological transport agent fQr transporting pharmacological or chemotherapeutic a~ents to the central nervou system.

In order to allow the transport of a medicine to the central nervouS ~ystem by means of the agent according to the invention,this medicine must be bound to the thiolated polypeptide compound,employed as a transport agent, without of course modi~ying the pharmacological property of the medicine or the property of the II~ frayment to be 25 bound to the sPecific receptors of the tetanus toxin in the central nervous sy~tem The term "Medicines" is intended to desi~nate,according to the ~nvention~any substances having pharmacological properties,such as pharmacological agents,chemotherapeutic agents and the like. The ~edicines 1~

hich may be bound accord1ng to the Lnven-tion to the polypeptide compound 0mployed as a neuropharmacological transport agerlt must have ~NH2 group~.
As examples of medicines which may be transported 5 to the central nervous system by mean~ of the thlolated polypeptide compound acoording to the inven-tion, there may be mentioned:alkaline phosphatase,the A fragment of cholera tox;n,the A fragment of diphtheria toxin, dipyrido-indoles according to French patent 77,11,148 and, generally,any medicine having -NH2 groups.
It i~ known that the cholera toxin is bound to the GMl ganglioaides of the intestine wall and that the A
fragment is respon~ible for the increase in the cyolic AMP rate (oyclio adenosine-monophosphoric acid). On the lS other hand,in ketanus a deorease in the cyclia ~MP
proportion in the central nervous system is ~ound. The oonJugate according to the inYention,formed by the thiolated polypeptide compound coupled to the A fragment may be employed ~or ~ontrolling tetanu~.
2U The dipyrido-indoles aocording to French patent 77)11)148 are chemotherapeutic agents of utility in the treatment of cancers. In this fieldJ it is kno~n that metastases are due to the fact that the cancerous cells come to nestle in the central nervous system hen~e they migrate to other regions of the body,where they develop tumors.

The development oF metasta~es could be avoided or reduced provided that the mean for destroying these cells in the central nervous sya-tem can reach the oentral nervous system.
In the same way,the inven~ion may be applied to the treatment of cerebral tumor~, The present invention con~equently also relates to the means for coupllng the thiolated polypeptide compound acoording to the invention to medicines.
The means ~or coupling the compound according to the invention and ~he medicine to be transported use at least one disulphide bridge or at least one sul~ur irreYersible link~
The present invention therefore al~o relates to IIC
Pragment medicine conJugates compri~ng at lea~t one disulphide bridge or at least one sulfur irreversible link.
It is known to prepare protein con~ugates by formation of an intermolecular disulphide bridge. The formation of suoh an intermolecular disulphide bridge i achieved for example by reaction of a protein having thiol groups with a protein having dithiopyridyl groups.
For example, according to the process described by TE PIA0 I~ING et al / Biochemistry vol~17 n8,1978 J ~
two d~ferent proteins may be coupled by first binding thiol groups to one of the proteins and 4 dithiopyridyl groups to the other protein and by reacting the resulting 1~

modlfied pro-te~n~ under suitable conditlons in order to ~orm a disulphide bridge and to eliminate 4-thiopyridone.
The thiol groups may be bound to one o~ -the proteins by means of 4-methyl-mercapto-butyrimidate and the 4-S dithiopyridyl groups to the other protein by means of,for example7 3-methyl-(4'-dlthlopyrldyl)proplonimidate.
This coupling process produoes a protein-protein conJugate in whioh the fraction between the two proteins i symmetrical with re peot to the di ulphide bridge.
According to CARLSSON et al. (Bioch. ~., 1978~
173 , 723-724)the thiol group can be introduoed in one of the proteins by reac~ion o~ said proteln with N-succinimi-dyl-3~(2-pyridyl~dithio)propionate and subsequent reduction;acoording to this process9 the same reagent, namely the N-sucoinimidyl-3-(2-pyridyl-dithio)propionate, is used for intxoducing both thiol and dithiopyridyl groups in the proteins~ The resulting conJugates al~o have a binding fraction which i ~ymmetrioal relatiYe to the di ulphide bridge.

4-methyl-meroapto-butyrimidate has also been usad for ~orming higher dimer~ and oligomers of proteinc O~
30 S ribosome of Escllerohia Coli ~Biochemistry,12, 3266-3273,1973) The conJugates thus obtained have many applications,~or example a5 immunological assay reagents.
The procecs according to the invention for coupling the thiolated polypeptide oompound used as a neuropharmaoological transfer agent with a medicine by means of disulphide bridges comprises the ~teps of:
1)-introducing dithiopyridyl groups in the medicine to be bound;
2~-reacting the medicine having he dithiopyridyl groups with the thiolated polypeptide compound according to the invention.
The reaction diagram of ~his eoupling prooess may be repre~ented in the following manner when the dithiopyr~dyla-tion âgent used in step 1 is N-sucoinimidyl-3-(2-pyridyl-dithio)propionate:

l)M NH + ~ - S - S-CH2 - CH2 - C - 0 -M-NH ~ H2-CH2-S~ S- ~ + H0 o ~IIc~7-NH-rllntcH2)3J~H ~ M-~`lH-~_tH~ 2-S-S_~

M_NH_C_CH2_CH2_S_S_(~2)3-C-NH rIIo J~ ~ S

~ H
In this procees,another dithiopyridylation agent may be employed 9 such as dithiopyridine or any other agent suitable for suoh a reaotion.

Another Wdy of coupling the polypeptide compound,used as a neuropharmacologioal tran~port ~gent acoording to the invention consists of creating an irrever~ible llnk bet~een said agent and the medicine to be transported~
This process may be representerl by the following reactlon diagram:

1 ) 11-NH2 + ~

M _ NH ~

23 M - NH _ ~ ~ N ~ + ! IIc_7-NH~f~ C (CH2~3 ~ SH
NH2 r Cl M - NH - ~ ~ ON~ NH2~ J Cl o~ H2)3-ll-NH ! IIC-7 It compri~es the steps of:
l)reacting the medicine to be bound with the ester o~ metamaleimidobenzoyl-N-hydro%y-succinimide;
2)reacting the re~ulting cQmpound with the polypeptide compound according to the invention~

25 The foregoing reaction diagrams and those which will be giyen hereinafter are simplified and do not take into account the number of SH groups which may be bound to the IIC fragment.

It ~as already propo ed to use the ester of metamaleimidobenzoyl-N-hydro~y-succinimide for forining en~yme~ntlbocly con~ugates (FEBS Letters, v01.959 n2,Nov~ 197~). However,the ~eachings of the prior art did not permit to ~oresee that the use of the es-tsr of metamaleimidobenzoyl-N-hydrox~uccinimide for coupling the neuropharmaoologioal transport agent according to the invention to medicines would not modify or inhibit the pharmacological properties of said medicine and the property of the II~ fragment to be bound to the specific receptors oF the tetanus toxin in th0 central nervous system.
It will be observed that the coupling acoording to the invention of the medioine to the thiolated polypeptide compound employed as a neuropharmacological transport agent is achieved by known e4nventional techniques of protein~protein coupling. HoweYer,it should be noted that not all oF the proteln-protein coupling processe~
available to one skilled in the art are suitable for the purposes of the invention.Indeed~only the coupling prooesses which achieve a disulphide bridge or a sulfur irreversible link are suitable. In particular,it will be mentioned that the most conventional coupling process, which employs glutaraldehyde~is unsuitable for the purposes of the invention,since the IIC fragment treated with glutaraldehyde loses its properties of axonal transport and of binding to the reseptors of the tetanus toxln in the central nervous system. Thus,the IIC
fragment to which carbonyl groups would have been bound,for example with glutaraldehyde,is un~uitable For the purposes of the lnvention.
Another applicatlon ot` the -thiolated polypeptide compound according to the invention,is the labelling of neuronal oells. Thus,the compound of the inventîon may be employed as a specific labeller of neuronal cells~ It i5 also suitable ~or preparlng immunological reagen-ts.For example an enzymatic reagent may be prepared from the thiolated polypeptide compound acoording to the invention and alkaline phosphatase. It wa~ found that the resulting conJugate had both binding power of the IIC fragment and khe enzymatic activity of the phosphatase. The compound aocording to the invent~on i~ also suitable as a retrograde transsynaptlc tracer The polypeptide compound of the invention is also appropriate to prepare a diagnosis reagent by coupling with a labelling molecule,such as an ant;body,for example a radioactive antibody.
The invention will be now described in more detail by means of ths examples illustrating the preparation of the thiolated polypeptide compound of the invention and the coupling of the latter with medicines. In all the examples9 use was made of the IIC fragment as de~ined hereinbefore.

~.
Thiolation o~ the II ~ 2-iminothiolane c Thiolation of the IIC ~ragment was achieved by means of the method described by Schramm et al. ! Z . Physiol.Chem.
1977,358 ; 137-139 7.

~21_ The IIC fragmen-t (1.5 mg) in so]utlon ln 50%
gly~erol(0.2 ml)was thiolated by iminothiolane (2.5 mg)in ~olution in 750~ul of 0.2 M triethanolamine HCl buffer, pH 8.5-9Ø The reaction mixture was maintained at room S temperature for 2 hours. The exoess of reagent was eliminated by filtration on a "SEPHADEX G-25" column. The thlolated polypeptide product so obtalned contained 2~7-SH
groups.
EX_MPLE 2 Thiolation of the II ~ -c teine thiolaotone.
The IIC fragment thiolation was effected by the procedure described by 5INGER et al. !-~ IAm. Chem~ Soc., 1960 82, 567-~71 7 To 4 mg of the IIC fragment in solution in 0.2 ml of water, there wa~ added the K2C03/NaHC03 buffer,pH 10.7(0.2 ml). A
nitrogen current was passed therethrough in order to expel the air. 0.2 ml of an ~0 mgll olution of N acetylhomocystei-ne thiolaGtone was added. The reaction mixture was maintained for two hours at 4C under nitrogen. The reaction was thereafter stopped by filtration on SEPHADEX ~-25. The thus obtained product contained 4-SH groups~

25 com~ound /A fra ment of the di hteria toxin .
5 mg of the A fragment of diphteria toxin were used in 1.5 ml of 0.1 M phosphate buf~er containing 0.5 M NaCl (pH 7.5). There was added 312 ~g of N-succinimidyl-3-(2-pyridyl-dithio)propiona-te (Pharmacia~dissolvecl ln 25 ~1 o~
99~5~ ethanol (~lerck). After reaoting for 30 minutes in a water bath at 23C~ the dithiopyridylated toxin wa separa-ted from the exce~s of reagent by filtration on ~Sephadex G 25" buffered wi~h the same buffer.
This prooedure achieved introduction of 1.~ 2-pyrldyl-disulphide grouping per mole of A fragment, The IIC fragment wa~ thiolated by reaction with 4-methyl-mercaptobutyrimi~ate accordlng to the method de~oribed by TE PIA0 ~ING et al.~Biochemistry7vol.17,N89 1978). 5,37 mg of II¢ dissolved in 1.5 ml of 0,025 M
borate buffer, pH 9.0 ,were reacted with 3 mg of the thiola-tion agent dissolved in 100 ~1 of methanol at 0C for 30 minùtes. The e~,aess of reagent ~as removed by filtration on "Sephadex G 25" balanced with a 0.1 M phosphate buffer~pH
7.0, containing 1 mM Na2 EDTA. The thiolated IIC fragment contained 2~5-SH groups per mole.
The conJugate was obtained by mixing 119 nanomoles of dithiopyridylated A fragment with 90.g nanomole~ of thiolated IIC fragment, The exchange reaction was followed at 343 nm. Filtration was effected on "Sepharose 6B"
buffered with a buffer Tris 0,05 M, 0,5 M NaCl,pH 8,0, The conJugate thus obtained preserves the binding power of the IIC fragment and the immunological reaotivity of 2S the A fragment and of the IIC fragment.
Figures lA and l,B illustrate the synthesis of the IIC-polypeptide conJugate. They represent the result of ~wo gel immunodif~usion experiments.

The photograph in the l~ft part of figure lA shows the specit`ic immunological reaction be-tween the TT
(-te-tanus toxin)fragment and the antltetanus serum disposed in the central well. No reaction occur~ with the A
fragment of the diphteria toxin. The A fragment of the diphteria toxin,which has an antigenic structure different from that of the tetanus toxin,exhib~ts no precipitation line when conta ted with antitetanus serum.
In figure lA,the ! A-SS-IIc 7 compound, viz. the thiolated IIo previously ooupled with the A fragment of diphterial toxln~is reacting with the antiteta~s serum ~ATS)through its tetanus antigenic determinants.
In figure lB9the L A-SS-IIc 7compoun'd is reacting with antidiph~eria serunl (ADS)through the antlgeni¢
determinants borne by the A fragment of ~he diphterial toxin.
Thus,the photographs in figures lA and lB show that synthe3is of A-SS_IIC conJugate occurred and that the two components, viz. the A fragment and the IIC ~ragment, retained their antigenic properties after the coupling.

~s In these exp~riments9 the IIC ~ragment,the thiolated polypeptide compound according to the invention,as well as the tetanic toxin and the other fragments used for comparison purposes~such as fragment B-IIb 9were labelled with 125 iodine. This labelling with 125 iodine was made according to th~ method of GREENWOOD et al.L The preparation of ~25I-labelled human gro~th hormone of high specific radioactivity, Blochem, ~, 8~,(19~3 114-127 7, The specific radioactivity of the 12~I-labelled IIo fragment used in the retrograde transport e~periments was of Lt ~Ci per ~9 of pro-tein. The IIo fragment used ~or testing the binding to synaptic membranes had a speoific radioactivity of 1,2 ~uCi per ~g of protein , The specific radioactivity of the tetanic toxin was of 2.5 ~Ci per ~9 o~ protein.
A)II~ fragment bindlng to ~r~esynaptic membranes _ ~___ Thecrud~ synaptic membranes were prepared from the spinal cord of male Sprague Dawley rats of a weight of 150-2009 aco4rding to the Young and Snyder method95trychnine binding associated with glyoine receptors of the central nervous system" ~r~c Na~_ A~d Scl. U.S.A, 70,(1973) 2832-283~. The protein concentration in the membrane ~uspension was adJusted to 1,0 mg per ml, The suspension was store~ in aliquots of 1 ml at -25C, The tests were effeoted aocording to the above-mentioned Young and Snyder method, The IIC, B-IIb fragments,or the toxin,were caused to reaot with aliquots of 0-1 mg ofcrude synaptic membranes for 15 minutes at 21C in 1 ml of 0.05 M Na/K
pho phate buffer, pH 7,4,containing 0,01~ of "Triton X-100"
The reaction was terminated by centrifuging during 10 minu-tes at 48~000 x g and 4C, The ~upernatent fluid wa deoanted and the sediment was washed three times with 5 ml of 0,05 M Na/K phosphate buffer, pH 7.4, containing 0.01%

~ 3P~

of "Triton X-100" and 0.1% of bovine -~erum albumin.The radioaotivity of the ~up~rnatent,of the washing liquors and of the eediment was then mea~ured. Each test ~vas repeated three times . The tests were al~o e-Pfected with unlabelled IIa or B-IIb fragments or with unlabelled toxln.
In this case~ a fixed amount of labelled protein was incubed with increasing amounts oF unlabelled proteins.
It was ascertained that binding of the 1~5I-labelled IIC
fragment was increasing linearly with the membrane oonoentration.
Displacement of the 125I-XIC fragment bound to the isolated synaptic membranes by th~ unlabelled II~ fragment or labelled tetanus toxin and displacement of the non 125I-labelled tetanus toxin by the unlabelled IIC or B-IIb fragments were mea.~ured. The results obtained are shown in figures 2A and 2B wherein there was plotted as ordinates the spe~ific binding o~ the 125I~IIC fragment (figure 2A) as expres~ed in ~pm (coun~ per minutej or of the 125I-labelled tetanus toxin (figure 2B)and a~ abscis ae the molar concentratîon (M) of IIo, B-IIb fragments or of tetanic tcxin.
The half maximal displacements were computed from the curve~ in figures 2A and 2B and are gathered in table I.
The results in table I show that the half maximal di.~placement of the 125I-IIC fragment bound to synaptic membranes has approximately the ~ame value whether it is caused by the unlabelled IIC fragment or by the whole tetanus toxin. The results in table I also show that the unlabelled IIo fragment is as much effective to displa~e 125I-labelled ¢~

tetanus toxi,n as the unlabel;Led -toxin i5 to displace the ~5I-IIC fra~m~nt' In oontrary,the B-IIb fragment 19 twice more e~fective than the IIC fragment to displace the 125 iodlne-labelled tetanus to%in bound ~o synaptic membrane~.
The Hill transformation o~ the displacemen-t o~ the 125I-IIC ~ragment bound to isolated synaptic membranes by the unlabelled IIC fragment,expressed as Lo~10 / Log10 100% - % B max of the concentra~ion of the unlabelled II~ fragment provides a linear ourve (flgu.re 3)with a ~lope o~ 1.68 whioh may be oonstrued as showing a positive cooperatiYity according to Cornish Bowden et al. 7 Diagnostic usea of the lS Hill(Logit and Ne~nst)~lotsD ~. Mol~ ~1O1.~5(1975) ~01 212.
The same tests as above were effected with the thiolated IIC fragment according to the invention obtained aoaording to th~ above Example I. The results obtained?al o shown in table I,show that thîolation of the IIC fragment aooording to the invention does not -~ub tantially modify the properties of fragment IIC to be bound to synaptic membranes.
B-A~onal_retro~rade trans~
~ e For all experiment~ 9 female Sprague-Dawley rats weighing 250g were u~ed. The rats were maintained at a constant temperature o~ 23C and fed with ~he usual diet (Nafage Go~sau)and water~

Two albino rats received an inJection of the ~SI-IIc fragment and of 30~ horse-radish peroxydase (HRP) As referenoes, two other rats received an inJection of the B-IIb fragment (1 ~1 corresponding to a ooncentrat~on of 0.7 ~9 of protein per ~
After baring the musole, the subJtances were inJeoted by an thermally con~rolled injection system by means o~ a glass pipette (50-100 ~um in out-~lde diameter)~ The inJection time wa about ~0 minutes~
The total inJected volume was 205 ~1, Ylz, 1.5 ~1 of IIC
fragment solution with 3 ~g of protein per ~l~and 1 lul o~ HRP
solution;
The rats were saorificed 24 hours after the inJection.
An intracardial perfu~ion was e~fec~ed under general anesthe-sia first with 0.5 ml of "Liquemine"~Roche) and 0 5 ml of sodium nitrite (OcOl g/ml~,then with 200 ml of a plasma expander ("MACRODEX") for 5 minute~,thereafter with 600 ml of lX para~ormaldehyde and 2.5% glutaraldehyde in a 0.1 M phos-phate buffer for 30 minute~,and la~tly with a 10% sucrose solution in a O.:L M phosphate buffer ~or 20 minutes.
For mor~ details about this procedure, reference may be made to the artiole o~ MESULAM ! ~. Hi~tochem. Cytochem., 2~ 78) 106-117 7.
The b~ain was removed immediately after this perfusion 25 and placed in a 30% sucrose solution for 48 hours be~ore being cut. Frozen sections (30 ~m in thickness~were taken~
starting ~rom the caudal end o~ the abducens nucleus up to the rostral end of the whole oculomotor nucleus.Each section treated with the horse-radi~h peroxidase was stained 2~

by the T~ method of MESULAM and r~stalned with neutral red while the other sectlons were autoradiographed~
The latter sections were mounted and dipped into a liquid emulsion NTB2 at 45C diluted 1:2 with distilled S water. lhe seotions were exposed for 4 weeks at 4C
in the dark and developed with "Kodak Dektol" developer at 18C for 90 seoonds,washed,and then ~i%ed with 30% ~odium thiosul-fate~washed for 2 hoursythen stained with cresyl violet and covered. All the ceotions were then examined under the microsoope (~50 magni~ication)and the location of the labelled cells was asoerta~ned by microphotography~
As the B-IIb ~ragm~nt,the II~ fragment undergoe~
axonal retrograde transport since positive labelling oocurs in the oculomotor nucleus of the two tested animal a~ter lS the inJection o~ HRP and of the IIC fragment,as evidenced by photographs of the ocul3motor complex with a radioactive labeller taken in the region under consideration.
.The main difference between the HRP and the IIC
20 fragment re3ide in the faot that the location of the HRP
granules is llmited to the perikarya and dendrites o~ the ooulomotor neurones,whlle the silver grain~ which represent the 125I-II fragment were also found in the pericellular spaces.

As compared with the B_IIb ~ragment and conJugates thereof,the IIo fragment seems to be more stri~tly located in the cellular bodies of the labelled motor neurones than in the extra cellular spaces. While a higher total actiYity 2~

was inJected in both cases,with the IXC there was noted a less intenslve labelling o~ the oculomo-tor nucleus than with the B-IIb fragment.
The fact that the axonal retroyrade transport is in direct relation~hip w~th the ability oF the fragments to be bound to their membrane reaeptors clearly shows that the $hiolated IIo fragment which ha kept his binding properties (Table I)also undergoes retroyrade transport.
By means of the te~ts described in the ~ournal of Neurochemistry(1977)vol,28 , p.529-542, it was ~erified that the IIG medioine conjugates obtained according to the invention ~rom the thiolated polypeptide oompound were bound in a ~pecific manner to the gangliosides and the ~ynapt.io membrane~;these re~ults demonstrate that these ~ubstance can consequently be transported in retrograde axonal manner.
TAELE I
hal~ maximal ~ r~ of proteins _f membrane a 20 ~E~ h g 1~5I~ 8 . o unlabelled IIC fragment 125I ~ 9.0 unlabelle8 tetanic toxln 125I _ tetanic toxin- 9.0 unlabelled IIC fragment 125I - tetanic toxin- 5'5 unlabelled B-IIb fragment I- thiolated I-II 9.5 unlabelled IIC fragmeCnt

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for thiolating IIc fragment of tetanus toxin comprising reacting said fragment with a thiolating agent, the thiolating agent and reaction conditions being selected such that the thiolated fragment includes one or more Z-SH groups, Z being the residue of the thiolating agent, has retrograde axonal transport properties, and is capable of binding to specific receptors of tetanus toxin in the central nervous system.

2. A process according to claim 1 wherein Z-SH is selected from and .

3. A process according to claim 2 wherein the thio-lating agent is selected from the group consisting of:
4-methylmercaptobutyrimidate, 2-iminothiolan , N-acetylhomocysteine thiolactone, and S-acetylmercaptosuccinimic acid anhydride.

4. A process according to claim 1 wherein each thiolated IIc fragment comprises from 1 to 5 Z-SH groups.

5. A thiolated IIc fragment of tetanus toxin prepared by the process according to claim 1.

6. A thiolated IIc fragment of tetanus toxin prepared by the process according to claim 2.

7. A thiolated IIc fragment of tetanus toxin prepared by the process according to claim 4.